The overhead projector is a didactical appliance which has found widespread application during the last decades. As opposed to a normal slide projector, the light in the overhead projector runs vertically and is not diverted into the horizontal until it is near the projection objective. The plane of the slide is, therefore, horizontal and is easily accessible. The image reversal by the deviating mirror results in the fact that a slide, which gives a true-to-side image on the projecting wall, is also true-to-side as seen from above. Since additionally, a very large slide format compared to normal slide projection is chosen (mostly between 25.times.25 cm and 30.times.30 cm), it is easy to draw or write on the slide during projection (hence the name "working projector" which is sometimes given to this unit). The size of the slide allows an optical construction which uses the light output of the projection lamp so well that the projected image is sufficiently bright with lamps of 250 to 500 watts so that complete darkening of the room is no longer necessary. The listeners can thereby make notes during the lecture. A further advantage of the overhead projector is the fact that the person working with the unit looks in the direction of the listeners (which is not normally the case when writing on a black-board or when projecting normal slides). He can, therefore, watch the faces of the listeners and adapt the lecture to their reactions.
Usually, clear plastic films (mostly made of cellulose acetate) are used as image carriers for the slides which are written or drawn upon during the lecture. They are written on with black or colored felt-tipped pens or similar writing and drawing markers. This procedure has several disadvantages:
the writing or drawing marker is filled with a black or colored solution. When marking with them it can therefore easily happen that hands, clothing as well as the slides are smeared with this. PA1 the legibility of a black or colored marking on the white background of the projected image is not especially good. Negative writing i.e. light marking on a dark background is more easily legible. The discernableness is even better if a color contrast is given in addition to the lightness contrast i.e. the light and dark parts of the image are not in the same color shade. Statistical tests have shown that the best legibility is achieved with light yellow marking on a dark blue background.